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Hesperosaurus
Hesperosaurus (meaning "western lizard", from Classical Greek, ἕσπερο-/hespero- "western" and σαυρος/sauros "lizard") was a herbivorous dinosaur from the Kimmeridgian to Tithonian epochs of the Jurassic period (approximately 150 million years ago), whose fossils are found in the state of Wyoming in the United States of America. It is from an older part of the Morrison Formation, and so a little older than other Morrison Stegosaurs. Discovery and Species It was described in 2001 by Kenneth Carpenter and colleagues, who took the name for its location in the western United States. A nearly complete skull and much of the skeleton was found. The only species known is Hesperosaurus mjos. It is present in stratigraphic zone 1 of the Morrison Formation.1 Susannah Maidment and colleagues proposed in 2008 that'' Hesperosaurus'' should be considered a synonym of Stegosaurus, with H. mjosi becoming Stegosaurus mjosi. Description Hesperosaurus is a large stegosaurid. In 2010, Gregory S. Paul estimated its length at 6.5 metres, the weight at 3.5 tonnes.7 In 2001 Carpenter provided a diagnosis. Due to his conclusion that Hesperosaurus were rather basal, in it many comparisons were made with the basalmost known stegosaurian Huayangosaurus,1 that lost their relevance once it became clear that the phylogenetic position was in fact quite derived. In 2008 Maidment indicated three autapomorphies: the possession of eleven back vertebrae; the forth sacral not being fused to the sacrum; back plates that are longer (from front to rear) than tall. Maidment also provided some traits in which Hesperosaurus was more basal than Stegosaurus armatus. In the atlas, even in adult specimens the neural arches are not fused to the intercentrum. The postzygapophyses, the rear joint processes, of the rear neck vertebrae do not prominently protrude upwards. In the back vertebrae, the neural arches, above the level of the neural canal, are not especially lengthened to above. At the hip region ossified tendons are present. The ribs are expanded at their lower ends. The neural spines of the tail vertebrae are not bifurcated. The lower end of the pubic bone is expanded (spoon-shaped in side view).5 To Carpenter this differential diagnosis was problematic because he considered Stegosaurus armatus, the type species of Stegosaurus, a nomen dubium and rejected Maidment's lumping of all North-American Stegosaurus material into a single species, the great variability of which making it difficult to establish any differences with Hesperosaurus. He considered Stegosaurus stenops, the name historically given to several well-preserved specimens, a separate species and provided a new differential diagnosis of Hesperosaurus compared to S. stenops. The antorbital fenestra is large instead of very small. The maxilla is short and deep, half as tall as long, instead of having a height a third of the length. The basisphenoid of the lower braincase is short instead of long. Thirteen neck vertebrae are present instead of ten. Thirteen dorsal (back) vertebrae are present instead of seventeen. The middle dorsals have a basal form in possessing a low neural arch rather than a high one. The cervical ribs have expanded lower ends. In the front tail vertebrae, the tops of the neural spines are rounded instead of bifurcated. The front edge of the shoulderblade is indented instead of running parallel to the rear edge. The front blade of the ilium diverges strongly sideways instead of weakly. The rear blade of the ilium has a knob-shaped expansion at the rear end. The front end of the prepubic process has an upward expansion. The plates of the hip and tail base are oval and low instead of high and triangular.6 The various published descriptions of Hesperosaurus contradict each other because of changes and differences in interpretation. Originally, Carpenter reconstructed the disarticulated skull elements into a very convex head, modelling it on the shape of Huayangosaurus.1 The discrepancies in the vertebral count are caused by applying different criteria to the problem whether (and which) cervicodorsal vertebrae should be considered part of the neck or the back. The exact shape of the plates is hard to determine due to erosion. Paul considered the neck plates to be low, but the back plates as taller.7 Also the Aathal specimens are as yet undescribed. A complete description of the entire material is in preparation by Octávio Mateus.4 The number of maxillary teeth were twenty per side, lower than the number with Stegosaurus. Carpenter described them as similar to the teeth of Stegosaurus, though somewhat larger.1 Peter Malcolm Galton in 2007 established some differences: there are rough vertical ridges present on the upper part of the crown, one per denticle; the fine grooves on the tooth surface are weakly developed. Osteoderms and skin impressions Carpenter in 2001 identified ten plates as part of the holotype. He described them as long and low. Asymmetrical bases would indicate that they ran in two rows. The end of the tail bore a "thagomizer" of two pairs of spikes, the front pair being thicker, the rear pair thinner and more horizontally directed to behind.1 In 2012, an histological study concluded that these osteoderms, skin ossifications, of Hesperosaurus are essentially identical in structure to those of Stegosaurus. CAT-scans showed that the plates have thin but dense outer walls, filled with thick spongy bone. The bone shows signs of having been remodelled during a metaplastic growth process. Extensive long and wide arterial canals were visible. The spikes have thicker walls and the hollows in the spongy interior are smaller. A single large blood vessel ran along the longitudinal axis of the spike.9 In 2010, a study was published on the soft parts visible with the "Victoria" specimen. It preserves both true impressions of the skin into the surrounding sediment, and natural casts, where the spaces left behind by the rotting of the soft body parts have been filled in with sediment. Additionally on some areas a black layer is present, possibly consisting of organic remains or bacterial mats. A part of the lower trunk flank shows rows of small hexagonal, non-overlapping, convex scales, two to seven millimetres in diameter. Higher on the flank two rosette structures are visible with larger central scales, one being twenty by ten millimetres in size, the other ten by eight millimetres. Apart from the scales, an impression of the lower side of a back plate has been found, covering about two hundred square centimetres. This shows no scales but a smooth surface with low parallel vertical ridges. As it is a true impression, with the life animal grooves would have been present. These grooves would have been about half a millimetre deep and stood about two millimetres apart. The impression probably represented the horn sheath of the plate, as would be confirmed by vertical traces of veins. It is the first direct proof of such sheaths with any stegosaurian. The study considered the presence of a sheath to be a strong indication that the plate had primarily a defensive function, as a horn layer would have strengthened the plate as a whole and provided it with sharp cutting edges. Also the display function would have been reinforced, because the sheath would have increased the visible surface and such horn structures are often brightly coloured. Thermoregulation on the other hand, another often assumed rôle of the plates, would have been hampered by an extra insulating layer and the smoothness of the surface, but cannot be entirely ruled out as extant cattle and ducks use horns and beaks to dump excess heat despite the horn covering. Phylogeny Category:Stegosaurs Category:Dinosaurs of North America Category:Jurassic dinosaurs Category:Morrison fauna